This invention relates to the manufacture of polyolefin tow for use in cigarette filters. In particular, this invention relates to a method and apparatus for improving the crimping of polyolefin filter tow.
It is known to produce polyolefin filter tow by slitting polyolefin film to fibrillate the film, and then subjecting the fibrillated film to a crimping operation. Crimping the fibrillated film gives it more bulk, and makes it "fluff up" so that it is more like traditional cigarette filter materials, such as cellulose acetate. One such polyolefin filter tow and its manufacture are described in U.S. Pat. No. 3,880,173.
The crimp imparted to the fibrillated film is characterized by a crimp frequency and a crimp amplitude. If a crimped fiber is compared to a sine wave, it has a certain number of crimps per inch (cpi) and each crimp has a certain amplitude. The crimp amplitude generally decreases as the crimp frequency increases.
When fibrillated polyolefin film is made into filter tow, and cigarette filters are made from the tow, the tow has a certain "yield", defined as the pressure drop obtainable from a given weight of filter tow. Yield may by measured, for example, in millimeters of water per milligram (mm WG/mg). It is desirable to maximize the yield from a given weight of filter tow. One known way of increasing the yield is to increase crimp frequency and uniformity.
It is also desirable to decrease the variability of the filter tow, in terms of density, so that every filter produced presents nearly the same feel to a smoker. Variability can also be decreased by increasing crimp frequency and uniformity.
It has heretofore been difficult to achieve significant improvements in crimp frequency. Crimp can be imparted to the fibrillated film fibers by gear tooth crimping, false twist crimping, or stuffer box crimping. The former two crimping methods suffer from known inherent mechanical limitations on the achievable crimp frequency. In stuffer box crimping, in which the fibrillated film is essentially rammed into an immovable wall, causing it to collapse, imparting crimp, there is no mechanical limitation, but the resilience of the fibers, and the stresses induced by the fibrillation process, make it difficult to increase the crimp frequency, and to achieve uniform crimping along a fiber.
It would be desirable to be able to increase crimp frequency and uniformity in fibrillated polyolefin filter tow and thereby to improve the yield and variability of the filter tow.